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Article

Effect of Annealing after Casting and Cold Rolling on Microstructure and Electrochemical Behavior of High-Entropy Alloy, Cantor

by
Jinsurang Lim
1,†,
Byung-Hyun Shin
2,†,
Doo-In Kim
3,†,
Jong-Seong Bae
2,
Jung-Woo Ok
2,
Seongjun Kim
2,
Jinyong Park
2,
Je In Lee
1,* and
Jang-Hee Yoon
2,*
1
School of Materials Science and Engineering, Pusan National University, Busan 46742, Republic of Korea
2
Busan Center, Korea Basic Science Institute, Busan 46742, Republic of Korea
3
Innovative Graduate Education Program for Global High-Tech Materials and Parts, Pusan National University, Busan 46241, Republic of Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Metals 2024, 14(8), 846; https://doi.org/10.3390/met14080846
Submission received: 3 July 2024 / Revised: 22 July 2024 / Accepted: 23 July 2024 / Published: 24 July 2024
(This article belongs to the Special Issue Casting Alloy Design and Characterization—2nd Edition)
Browse Figures
Versions Notes

Abstract

:
High-entropy alloys (HEAs), a relatively new class of materials, have attracted significant attention in materials science owing to their unique properties and potential applications. High entropy stabilizes the phase of a solid solution over a wide range of chemical compositions, yielding unique properties superior to those of conventional alloys. Therefore, this study analyzed the microstructure and electrochemical behavior of HEAs (Cantor) to evaluate their corrosion resistance, according to their manufacturing process (casting, cold rolling, and annealing). The microstructural morphologies and sizes were analyzed using electron backscatter diffraction. The electrochemical behavior was examined using open circuit potential measurements, electrochemical impedance spectroscopy, potentiodynamic polarization tests, and critical pitting temperature measurements using a potentiostat. The casting process formed a nonuniform microstructure (average grain size = 19 μm). The cold rolling process caused the formation of fine grains (size = 4 μm). A uniform microstructure (grain size > 151 μm) was formed after heat treatment. The corrosion resistance of the HEAs was determined from the passivation layer formed by Cr oxidation. These microstructural differences resulted in variations in the electrochemical behavior. Microstructural and electrochemical analyses are crucial because HEAs have diverse potential applications. Therefore, this study contributes to future improvements in HEA manufacturing processes.

1. Introduction

Metallic materials including iron have been developed into approximately 30 alloy systems by several researchers in the 1970s [1,2,3]. However, further research is required for their use in changing environments. Despite the development of many alloys, manufacturing difficulties have hindered the commercialization of metal alloys [1,4,5]. Recent advancements in manufacturing technologies such as heat treatment, cooling, shaping, and extrusion have enabled the development of various metal alloys [6,7,8]. Consequently, new alloys are being developed, resulting in the emergence of new manufacturing techniques and materials. The development of new alloys is closely related to the advancements in manufacturing technologies. Controlling the phase transformations caused by differences in alloy composition using existing technologies is challenging. However, recent technological advancements such as powder metallurgy, vacuum casting, and heat treatment have facilitated the development of various alloys. Consequently, diverse alloys continue to be developed to metallic materials in general.
Existing alloy systems typically comprise one to three alloying elements [9,10,11]. They rely primarily on substitutional alloys to induce variations in properties. Among the most commonly used steels, transformation induced plasticity (TRIP, added Si) steel enhances the formability of the material through the addition of Si. Meanwhile, twinning induced plasticity (TWIP, added Mn) steel enhances the strength and ductility by the addition of Mn. The mechanical and electrochemical properties of these materials have diversified with the development of various alloys [12,13,14]. As various characteristics emerge depending on the chemical composition of the alloy, the development of new functional alloys is a crucial task in response to the need for carbon emission reduction and energy conservation. Additionally, studying the electrochemical properties is essential for the utilization of advanced development materials.
Among various functional alloy metals, high-entropy alloys (HEAs) have been developed recently [15,16,17]. Herein, the major alloying elements are mixed in equal proportions. These alloys exhibit properties that differ from those of conventional alloys. As the name indicates, HEAs have high entropy [18,19,20], which stabilizes the microstructure. Stable microstructures enhance the strength, hardness, and corrosion resistance of alloys [6,7,21]. Therefore, stabilized HEAs can be used in various industries, such as the mechanical, aerospace, space, chemical, electrical, and electronic industries. Owing to these characteristics, HEAs have attracted significant interest in advanced engineering and scientific fields. This has led to continuous research and development in this area.
Cantor (NiCoCrMnFe) is one of the most popular HEAs [6]. As proposed by J.W. Cantor in 2004, it comprises five alloying elements mixed in equal proportions. This composition maintains the lattice structure of Cantor as face-centered cubic (FCC) [7,8]. Cantor exhibits remarkable strength, ductility, corrosion resistance, high-temperature stability, thermal fatigue resistance, and low-temperature impact toughness, rendering it suitable for use in extreme material environments. However, research on the optimization of corrosion resistance based on manufacturing processes is insufficient. Polar regions are exposed to exceptionally low temperatures in winter, whereas equatorial regions encounter significantly higher temperatures in summer [22,23,24]. The equatorial regions are suffering from various problems due to higher summer temperatures than before [25,26,27]. For use in various environments, structural materials must have excellent physical and electrochemical properties. Among the various materials available, Cantor is suitable for use in such environments. However, research on the electrochemical properties of Cantor is limited.
Therefore, this study analyzed the microstructure and electrochemical behavior of Cantor (Cr20Mn20Fe20Co20Ni20, each 20 at.%) according to its annealing after casting and cold rolling to determine its utilization as a structural material. The manufacturing process of Cantor involved casting, rolling, and annealing. The resulting microstructure and chemical composition were analyzed using electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS). Furthermore, the electrochemical behavior according to the process was analyzed using open circuit potential (OCP) measurements, potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS), and critical pitting temperature (CPT) measurements. The passivation layers on the surface of the HEA were analyzed using X-ray photoelectron spectroscopy (XPS). This study evaluated the microstructure and corrosion resistance of HEAs according to their manufacturing process and compared the corrosion resistance with stainless steel AISI304 and AISI316 (POSCO, Pohang, Republic of Korea).

2. Materials and Methods

2.1. Materials

The manufacturing process conditions (casting, cold rolling, and annealing) of the HEA are shown in Figure 1. The HEA was melted and produced in a high-frequency vacuum furnace. Cantor (20 at.% (18.5 wt.%) Cr, 20 at.% (19.4 wt.%) Mn, 20 at.% (19.9 wt.%) Fe, 20 at.% (21.1 wt.%) Co, and 20 at.% (20.9 wt.%) Ni, by inductively coupled plasma mass spectrometry, (ICP-MS, Thermo Fisher Scientific, Waltham, MA, USA)) was cast in a high-frequency vacuum furnace under an argon gas atmosphere for 24 h, followed by water quenching (#a, black line) [6,16,17]. After casting, 1 kg and 3 mm thick slabs were fabricated for the other experiments. Subsequently, cold rolling was performed for thickness control; the thickness of the HEA material was reduced from 3.0 to 1.5 mm, which represents a 50% reduction (#b, red line). Annealing was conducted at 1100 °C (#c, blue line) to achieve retained stress relief and property enhancement, followed by quenching in water [12,28,29].

2.2. Microstructure

The microstructure and average grain size of the Cantor were analyzed according to the manufacturing conditions using EBSD (SUPRA 40VP system, Zeiss; Oberkochen, Germany). The surfaces were polished, starting with emery paper (#200–#2000) and progressing to colloidal silica (0.25 μm). An electrolyte solution of ethanol (78 mL), hydrochloric acid (18 mL), and nitric acid (4 mL) was used for microstructural examination. The EBSD images were examined at magnifications of ×1000 and ×200 seven times to observe the variations in grain size due to the manufacturing process.

2.3. Electrochemical Behavior

The electrochemical behavior was measured in a three-electrode cell using a potentiometer (Versa State 4.0; Princeton, NJ, USA) [30,31,32]. The cell consisted of a working electrode (specimens, calculation area of 100 mm2), counter electrode (20 mm × 20 mm Pt mesh), and reference electrode (saturated calomel electrode (SCE), KCl electrolyte solution). The electrolyte solutions were prepared according to the ASTM G 61 and ASTM G 150-99 standards, utilizing 3.5 wt.% NaCl and 5.58 wt.% NaCl solutions [33,34,35,36]. To examine the electrochemical test results, the specimen surfaces were polished with colloidal silica to reduce the effects of surface state.
Electrochemical analyses were conducted using OCP measurements, potentiodynamic polarization tests, EIS, and CPT measurements. Pure metals permit the calculation of their potential, whereas this is more challenging for alloys owing to galvanic corrosion and the passivation layers [37,38,39]. Therefore, to assess the reactivity of HEAs under different manufacturing conditions, their potentials were measured using the OCP. The OCP was measured over time, with potential variations recorded for up to 3600 s at a scan rate of 0.5 s/point. Potentiodynamic polarization tests enable the observation of corrosion behavior by measuring the variations in the current density corresponding to the potential (corrosion reaction). The potential was varied from −0.6 to 1.2 V with a scan rate of 0.167 V/s. EIS measures the variation in resistance with frequency to analyze the state of the passivation layers present on the surface (e.g., the Cr oxide layer or the plated layer of stainless steel). The frequency range for EIS was 106–100 Hz. As the Cantor contained 20 at.% Cr, the Cr oxide layer (passivation layer) was examined using EIS. The states of the microstructures and passivation layers generated during the manufacturing process were assessed. The passivation layer in Cantor determines its corrosion resistance. The passivation layer in stainless steel can be evaluated using the CPT and pitting resistance equivalent number (PREN) [27,40,41]. The PREN of the materials used in this study contains 18.5 wt.% Cr. This allows for the calculation of Cantor’s PREN as 18.5 according to the following formula. Although the materials used in this study had compositions identical to that of Cantor, the differences in the microstructure due to the manufacturing process affected the CPT. The CPT test was conducted in a 5.85 wt.% NaCl electrolyte solution at a heating rate of 1 °C/min. The CPT is determined by exceeding a temperature that results in a current density of 100 μA/cm2 for 1 min.

2.4. Analysis of Surface

The surface condition was analyzed using XPS and EDS. The surface of the HEAs was polished with colloidal silica and subsequently cleaned. The binding energies were measured using XPS (K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA). Analyses were conducted according to the manufacturing process used to observe the effects of annealing. The surface area of the specimen analyzed using XPS was 5 mm × 5 mm. After polishing the surface before and after the corrosion test, the surface components were analyzed using EDS. The analysis was repeated five times, and the average was calculated.

3. Results

3.1. Microstructure

The microstructure of the Cantor alloy was examined using EBSD to assess the effects of the manufacturing process [6,15,16,17]. The results are shown in Figure 2. The EBSD images revealed variations in the grain morphology and grain size of the microstructure according to the manufacturing process. Despite the variations in grain size and morphology, the microstructure of Cantor remained FCC. The cast microstructure of the HEA exhibited an average grain size of 19 μm. After the cold rolling, the thickness decreased by 50%, and the grain size decreased to an average of 4 μm (decreasing rate of thickness, 79%). After annealing of the microstructure of the HEA, the grain size increased dramatically from an average of 4 to 151 μm. Thus, there was significant grain growth. This result shows the grain growth due to recrystallization.
This growth indicates recrystallization, which was likely due to stress relief. Additionally, after annealing, the Cantor exhibited up to six twins within the large grains. This indicates a strengthening mechanism, which can be attributed to the slip systems induced by Mn. The grain coarsening of the HEA due to annealing demonstrates the stabilization at elevated temperatures, which is particularly evident at 1100 °C. This correlates with the microstructural stability of HEAs [8,18,19]. The grain size of conventional metals typically ranges from 20 to 40 μm [42,43,44]. However, after heat treatment, the grain size of the HEA was more than four times larger than that of conventional metals. This increase in grain size is attributed to the stability of the structure due to the high entropy of HEAs [6,15,16]. Previous studies have reported that large grains can decrease corrosion resistance. However, electrochemical research on the effects of the grain size on HEAs is limited.
The manufacturing conditions affected the chemical composition and surface state of the HEAs. Table 1 shows the surface chemical composition of the HEAs according to the manufacturing process. The alloy segregation due to the manufacturing conditions influenced the formation of the surface oxide layer. The surface of the HEAs formed an oxide layer (Cr2O3) due to Cr, which retained a high amount of oxygen. High oxygen levels were observed after cold rolling, indicating significant surface oxidation. Cold rolled HEAs had a high density of dislocations and fine grains, which acted as factors accelerating surface oxidation. After solution annealing, the stabilized microstructure delayed surface oxidation and showed the lowest oxygen concentration. The processing conditions affected the oxidation of the HEA, which could influence its corrosion resistance.
The size and morphology of the grains in the Cantor HEA vary depending on the manufacturing process [4,5,11], affecting its corrosion resistance. Inhomogeneous microstructures accelerate galvanic corrosion because of the differences in grain size. Meanwhile, finer microstructures increase the corrosion rates owing to the increased interfacial area. Coarse grains are susceptible to corrosion because of their high energy content. Therefore, these differences in microstructures resulting from the manufacturing processes necessitate quantitative data on the electrochemical behavior of HEAs to understand their corrosion resistance more comprehensively.

3.2. Electrochemical Behavior

To evaluate the corrosion resistance of Cantor according to the manufacturing process, the electrochemical behavior was examined using OCP measurements, potentiodynamic polarization tests, EIS, and CPT measurements [45,46,47]. The OCP measurements were obtained to monitor the potential over time. The results are shown in Figure 3. The potential varied depending on the manufacturing process. The process-specific and alloy potentials are listed in Table 2. The main alloy potentials ranged from −0.23 to −1.05 V, lower than those of HEAs.
The differences in the HEA potentials originate from variations in the Cr passivation layer [48,49]. This is similar to the electrochemical behavior of stainless steel. Stainless steel forms a passivating Cr oxide layer on its surface [1,48], and this increases its potential. The behavior of stainless steel varies depending on its chemical composition, microstructure, and surface treatment with Cr. HEAs contain 18.5 wt.% Cr. A passivation layer is formed owing to Cr oxidation, which results in the enhanced potential observed in the OCP.
The variation in the potential of HEAs according to the manufacturing process is attributed to the residual stresses caused by processing and the differences in alloy precipitation [30,32,50]. Alloy precipitation, particularly of Cr and other elements, disrupts the uniform formation of the Cr oxide layer [1,48,49,51]. This reduces the potential. Residual stresses generated during processing facilitate the initiation of corrosion. The potentials of the cast HEAs reflect the influence of the passivation layer and alloy precipitation [48,49]. Meanwhile, those of the cold rolled HEAs are influenced by the passivation layer, alloy precipitation, residual stresses, and refined microstructures. The annealed HEAs were potentially influenced by the passivation layer and the coarsened microstructure. The differences in potential among the three HEAs due to the different processes contributed to the variations in corrosion behavior. Casted HEAs exhibit low potential (0.12 V) due to their heterogeneous microstructure and alloy segregation. Cold rolled HEAs show a decrease in potential (0.8 V) due to grain refinement and increased dislocation from rolling. After annealing, the recrystallized microstructure restores the potential (0.18 V) by decreasing the grain size, residual stress, and alloy segregation.
Potentiodynamic polarization testing measures the variations in current density with respect to the potential [1,12,13], indicating the variations in the corrosion rate due to the reactivity of the metals. The results of the potentiodynamic polarization testing according to the manufacturing process are shown in Figure 4 and Table 3. These exhibited a trend similar to that of the OCP measurements. During activation polarization, the casting material displayed a low potential (Ecorr) and high current density (Icorr). The low potential was due to the nonuniform morphology of the structure formed during casting. Anodic peaks appeared at 0.25 and 0.52 V, indicating the formation of a nonuniform passivation layer on the surface. The cold rolled materials exhibited lower potentials and higher current densities than the cast materials. This indicates the reduced corrosion resistance of HEAs. Cold rolling of HEAs causes a reduction in the corrosion resistance owing to grain refinement and increased residual stresses during processing. No pitting potential peaks (Epit) were detected, and the current density increased to 1 × 10−2 A/cm2 at 0.38 V [30,32,33], indicating uniform corrosion and the failure of the passivation layer [27,40,41]. Annealing revealed a nonuniform microstructure, grain refinement, and decreased residual stress, indicating structural coarsening.
Variations in the microstructure increased the potential and current density during activation polarization. The potential was reduced owing to the passivation layer. Meanwhile, the increase in the current density to 3 × 10−5 A/cm2 at 0.15 V is attributed to the increase in current density during the formation of the passivation layer [1,22,28,52]. The subsequent passivation prevented further increases in the current density up to 0.42 V. Following activation polarization, the pitting potential exhibited a trend similar to that of the corrosion potential. Processing affects the corrosion resistance of HEAs, which can be controlled by annealing. The Ecorr of the potentiodynamic polarization test represents the onset value of the oxidation reaction following the reduction reaction, which is a result of Cr repassivation. The repassivation shows a lower value than the OCP because it does not indicate an increase in the Cr passivation layer over time. The cold rolled HEAs exhibited a decrease of 0.11 V from 0.08 V to −0.03 V. The rolled microstructure is susceptible to the repassivation of the Cr passivation layer, which acts as a factor in decreasing the potential.
Despite an identical chemical composition, stainless steel has been reported to exhibit differences in surface passivation layers depending on the manufacturing process [29,39,51]. Given that HEAs exhibit microstructural variations depending on the manufacturing process, the influence of the passivation layer was examined using EIS. The results are presented in Figure 5 and Table 4. The HEAs exhibited differences in the EIS resistance based on the manufacturing process, as observed in the Bode and Nyquist plots. The Bode plot depicts the variations in the resistance and phase angle with frequency (Figure 5a,b). The Nyquist plot shows the results for the passivation layer (Figure 5c), indicating the differences in resistance based on the manufacturing process and reflecting the variations in corrosion resistance. The presence of a passivation layer on the surface was verified using the designed EIS circuit (Figure 5d). The circuit was structured to reflect the resistance of the passivation layer in HEAs [12,20,22].
The resistance of the passivation (Rp) layer varied based on the manufacturing process: the casting material exhibited a resistance of 6.1 kΩ, and the cold rolling material exhibited a resistance of 5.2 kΩ. After annealing, the resistance increased to 11.3 kΩ, signifying an increase in the thickness of the passivation layer and, hence, an enhancement in the corrosion resistance.
The variations in the microstructure according to the manufacturing process of the HEAs affect the EIS resistance and thereby the corrosion resistance [12,13]. The differences in the resistances of the passivation layers of the HEAs result in variations in the corrosion resistance; a higher resistance after heat treatment indicates a higher corrosion resistance, whereas a lower resistance in the cast and rolled materials indicates a lower corrosion resistance. These differences in resistance are primarily attributed to the nonuniform morphology of the cast microstructure, grain refinement, and residual stresses in the rolled structure. Annealing can strengthen the passivation layer through uniform grain morphology and size, coarsening of grains, and the relief of residual stresses [30,32,33]. The electrochemical behavior of HEAs is influenced by the passivation layer. Controlling the manufacturing process can regulate the corrosion resistance by controlling the grain morphology and size.
CPT tests were conducted to evaluate the performance of the passivation layers in the HEAs, and the results are shown in Figure 6. CPT testing is used to assess the lifespan of stainless steel passivation layers [1,31]. As HEAs display passivation layers similar to those of stainless steel, they were evaluated using the CPT test. The test was conducted according to the ASTM G 150-99 standards, and the results are presented in Figure 6 [31,32,33,34,35,36]. CPT represents the temperature at which the current density exceeds 100 A/μm2 for 1 min, which indicates a stable growth of the passivation layer [48,51,53,54]. The CPT values varied according to the HEA manufacturing process used. The sequence of CPT values was as follows: rolling (14 ± 1.1 °C), casting (12 ± 1.2 °C), and annealing (19 ± 0.6 °C). The differences in CPT based on the manufacturing process aligned with the trends observed in the potentiodynamic polarization tests and EIS results. The variations in the microstructure due to the manufacturing process influence the corrosion resistance, as evidenced by the CPT test. Therefore, to stabilize the corrosion resistance, HEAs require control over their microstructure because it significantly affects the corrosion resistance.
After conducting the CPT tests, we analyzed the images of the corroded surfaces shown in Figure 7. Corrosion occurred in a localized form, consistent with the pattern observed in stainless steel, owing to the formation of a Cr passivation layer. Thus, corrosion in HEAs exhibits resistance owing to the Cr passivation layer, and the corrosion growth proceeds during pitting. Pitting corrosion is a form of corrosion that occurs in metals forming a passivation layer. The passivation layer of HEAs consists of a Cr oxide layer and oxides of other metals. Therefore, the higher oxide layer of the cold rolled HEAs, being a mixture of the passivation layer and other metal oxides, had a detrimental effect on the corrosion resistance. The corrosion easily occurs at grain boundaries where dislocations are concentrated. The cold rolled HEA, with its refined grains, had numerous corrosion sites, which accelerated the growth of pitting corrosion.
The results of the corrosion tests indicate that the corrosion behavior of HEAs is similar to that of stainless steel. XPS, which reveals the surface composition and chemical state, is well-suited for analyzing the passivation layer on the surface. The results are presented in Figure 8. The primary surface components were found to be Cr and O, with Cr in the form of Cr2+ and O in the form of O−. The XPS results suggest the formation of a Cr2O3 passivation layer, which is consistent with the passivation layer observed in stainless steel [33,55,56,57].
The chemical compositions of the surfaces before and after corrosion were analyzed, and the results are presented in Table 5 [56,57]. Prior to the corrosion test, the surface formed an oxide layer and showed the lowest concentration of oxygen after heat treatment [48,49]. The surface of HEAs is composed of an oxide layer that acts as a barrier against corrosion. Although the surface was washed with distilled water after the corrosion test, removing the remains from the corrosion crevices was impossible, which allowed the prediction of the ions involved in the corrosion process. After the corrosion test, the surface exhibited corrosion caused by chlorine, which increased the concentration of O. In stainless steel, the destruction and repassivation of Cr during corrosion lead to a decrease in its concentration, which is consistent with the corrosion behavior observed in the HEAs in this study. The cold rolled specimen exhibited the lowest Cr concentration on the surface and the highest oxygen and Cr concentrations (highest corrosion). The corrosion led to a decrease in the Cr chemical composition, whereas an increase in the corroded area resulted in higher concentrations of O and Cr. The corrosion behavior of HEAs is similar to that of stainless steel, maintaining a passivation layer due to Cr [48,49]. However, the corrosion resistance varies depending on the condition of the material.

4. Discussion

HEAs exhibit microstructural stability owing to their high entropy. However, casting during manufacturing introduced heterogeneity in the microstructure. Cast microstructures exhibited a nonuniform grain size of 19 μm with dendritic formations. Cold rolling refined the grains to an average size of 4 μm. The subsequent annealing resulted in the formation of significantly coarser grains (average size = 151 μm) with twinning within the grains. This grain coarsening after annealing is attributed to the high entropy of the HEAs [6,15,18]. Therefore, HEAs can potentially enable microstructural control during manufacturing. This necessitates appropriate techniques for such control.
Cr formed a passivation layer on the surface, which increased the potential. This effect is similar to that in stainless steel, and the chemical composition of the major alloy affects the passivation layer. However, because the chemical composition of the HEA is the same, there is no potential difference due to the chemical composition, and differences in microstructure due to the manufacturing processes affect the potential.
The variability in corrosion resistance according to the manufacturing process is evident. The cast microstructures (characterized by heterogeneous microstructures) exhibited a low corrosion resistance. After casting, the cold rolled structures exhibited reduced corrosion resistance owing to the presence of finer grains and residual stress. Annealing-induced recrystallization enhanced the corrosion resistance, as indicated by the increase in CPT from 12 to 19 °C. The variations in the microstructural parameters (grain morphology, grain size, and residual stresses) due to the manufacturing process contribute to differences in the Cr oxide layer (passivation layer), thereby influencing the corrosion resistance [30,32,33]. The manufacturing process results in microstructural variations in HEAs, which affect the corrosion resistance differently. Although HEAs display structural stability owing to their high entropy, the microstructural differences originating from the manufacturing processes affect their corrosion resistance. This necessitates microstructural control of HEAs.
The corrosion resistance of the HEAs was determined based on the state of the passivation layer, similar to the case with stainless steel [1,30,48]. Stainless steel is distinguished by its corrosion resistance based on the PREN formula (Equation (1)). This accounts for the dependence of the chemical composition on the PREN [34,51].
PREN = wt.% Cr + 3.3 wt.% Mo + 16 wt.%
The PREN and CPT values of the HEAs and commercial AISI304 and AISI316 (austenite stainless steels) were determined according to the manufacturing process and are presented in Table 6. Although the corrosion resistance of HEAs varies depending on the manufacturing process, it is superior to that of AISI304 and inferior to that of AISI316. Therefore, HEAs can potentially replace stainless steel in various applications owing to their remarkable corrosion resistance, high strength, and superior low-temperature toughness. However, further research is required to obtain additional foundational data for their application. This includes (but is not limited to) corrosion resistance evaluation.
HEAs undergo passivation by Cr to form a passivation layer; however, the thickness of this layer varies depending on the manufacturing conditions. The corrosion behavior according to the difference in passivation layer thickness is illustrated in Figure 9. After passivation, corrosion occurred in the HEAs, taking on a pitting form. Differences in the thickness of the passivation layer resulted in variations in the corrosion rates, as confirmed by the CPT measurements. The corrosion process can be divided into two stages. The passivation layer is first disrupted by chloride ions, followed by the onset of corrosion on the substrate, which proceeds in a pitting manner. Although the pits may appear as small spherical dots on the surface, they form internally, significantly increasing the corrosion rate.

5. Conclusions

Owing to their high entropy, HEAs exhibit structural stability and can be used in various environments. However, the electrochemical data on HEAs under different processing conditions are insufficient. This study obtained foundational data on manufacturing processes. The following conclusions were drawn:
(1) The HEAs exhibited different microstructures after casting, cold rolling, and annealing (manufacturing processes). The cast microstructure exhibited nonuniform grain morphology and multidirectional twin growth. The cold rolled microstructure had finer grains, with an average grain size of 4 μm. After annealing, the microstructure of the HEAs exhibited recrystallization, resulting in an average grain size of 190 μm. A grain size of 190 μm in metals is exceptionally large. Such large grain sizes are attributed to the high entropy of the HEAs. The manufacturing processes influence the microstructure, which in turn affects the corrosion resistance.
(2) The electrochemical behaviors of the HEAs were analyzed (OCP measurements, potentiodynamic polarization tests, EIS, and CPT measurements) to evaluate their corrosion resistance. The HEAs exhibited an electrochemical behavior similar to that of stainless steel. This was verified by electrochemical tests. The corrosion resistance decreased in the following order: annealed, cast, and cold rolled. Although recrystallized, the annealed microstructure exhibited high resistance (11.3 kΩ) in EIS and a high CPT (19 °C) owing to its stable structure. Although nonuniform, the casted microstructure showed high resistance (6.2 kΩ) in EIS and a high CPT (14 °C). The cold rolled microstructure, with its finer grains and residual stress, showed low resistance (5.2 kΩ) in EIS and a low CPT (12 °C). Thus, the electrochemical behaviors of HEAs reflect those of conventional metals. Moreover, the microstructures of HEAs under different manufacturing conditions influence their corrosion resistance.
(3) The corrosion resistance of the HEAs was determined from the passivation layer formed by Cr oxidation, similar to the case with stainless steel. The PREN of the HEAs was higher than that of AISI304 and lower than that of AISI316. The CPT was in between. Therefore, the corrosion resistance of HEAs depends on the control of their microstructural states, which can be stabilized by annealing. HEAs exhibit an electrochemical behavior equivalent to that of stainless steel. Thus, these materials can potentially replace stainless steel in various applications.

Author Contributions

Conceptualization, J.L., B.-H.S. and J.I.L.; Methodology, B.-H.S.; Software, S.K., J.P. and J.-H.Y.; Validation, J.L., J.-W.O., S.K. and J.-H.Y.; Formal analysis, J.L., D.-I.K., J.-S.B. and J.-H.Y.; Investigation, B.-H.S.; Resources, J.L., B.-H.S. and J.I.L.; Data curation, J.L., D.-I.K., J.-W.O. and J.P.; Writing—original draft, B.-H.S. and J.-H.Y.; Writing—review & editing, B.-H.S., D.-I.K. and J.-S.B.; Visualization, J.L., D.-I.K. and J.I.L.; Supervision, B.-H.S., D.-I.K. and J.-H.Y.; Project administration, B.-H.S., J.I.L. and J.-H.Y.; Funding acquisition, D.-I.K., J.I.L. and J.-H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by grants from the Korea Basic Science Institute (grant number: C330320) and the BK21 FOUR program (grant number: 4120200513801), funded by the Ministry of Education (MOE, Korea) and the National Research Foundation of Korea (NRF).

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Nilsson, J.-O. Super Duplex Stainless Steels. Mater. Sci. Technol. 1992, 8, 685–700. [Google Scholar] [CrossRef]
  2. Tehovnik, F.; Arzensek, B.; Arh, B.; Skobir, D.; Pirnar, B.; Zuzek, B. Microstructure Evolution in SAF 2507 Super Duplex Stainless Steel. Mater. Technol. 2011, 45, 339–345. [Google Scholar]
  3. Linton, V.M.; Laycock, N.J.; Thomsen, S.J.; Klumpers, A. Failure of a Super Duplex Stainless Steel Reaction Vessel. Eng. Fail. Anal. 2004, 11, 243–256. [Google Scholar]
  4. Lervåg, M.; Sørensen, C.; Robertstad, A.; Brønstad, B.M.; Nyhus, B.; Eriksson, M.; Aune, R.; Ren, X.; Akselsen, O.M.; Bunaziv, I. Additive Manufacturing with Superduplex Stainless Steel Wire by Cmt Process. Metals 2020, 10, 272. [Google Scholar] [CrossRef]
  5. Moniruzzaman, F.N.U.M.; Shakil, S.I.; Shaha, S.K.; Kacher, J.; Nasiri, A.; Haghshenas, M.; Hadadzadeh, A. Study of Direct Aging Heat Treatment of Additively Manufactured PH13–8Mo Stainless Steel: Role of the Manufacturing Process, Phase Transformation Kinetics, and Microstructure Evolution. J. Mater. Res. Technol. 2023, 24, 3772–3787. [Google Scholar]
  6. Cantor, B. Multicomponent High-Entropy Cantor Alloys. Prog. Mater. Sci. 2021, 120, 100754. [Google Scholar]
  7. Otto, F.; Yang, Y.; Bei, H.; George, E.P. Relative Effects of Enthalpy and Entropy on the Phase Stability of Equiatomic High-Entropy Alloys. Acta Mater. 2013, 61, 2628–2638. [Google Scholar]
  8. George, E.P.; Curtin, W.A.; Tasan, C.C. High Entropy Alloys: A Focused Review of Mechanical Properties and Deformation Mechanisms. Acta Mater. 2020, 188, 435–474. [Google Scholar]
  9. Rani, K.U.; Kumar, R.; Mahapatra, M.M.; Mulik, R.S.; Świerczyńska, A.; Fydrych, D.; Pandey, C. Wire Arc Additive Manufactured Mild Steel and Austenitic Stainless Steel Components: Microstructure, Mechanical Properties and Residual Stresses. Materials 2022, 15, 7094. [Google Scholar] [CrossRef]
  10. Beziou, O.; Hamdi, I.; Boumerzoug, Z.; Brisset, F.; Baudin, T. Effect of Heat Treatment on the Welded Joint of X70 Steel Joined to Duplex Stainless Steel by Gas Tungsten Arc Welding. Int. J. Adv. Manuf. Technol. 2023, 127, 2799–2814. [Google Scholar]
  11. Fande, A.W.; Taiwade, R.V. Welding of Super Duplex Stainless Steel and Austenitic Stainless Steel:# Xd; Influence and Role of Bicomponent Fluxes. Mater. Manuf. Process. 2023, 38, 434–448. [Google Scholar]
  12. Rybalka, K.V.; Beketaeva, L.A.; Davydov, A.D. Electrochemical Behavior of Stainless Steel in Aerated NaCl Solutions by Electrochemical Impedance and Rotating Disk Electrode Methods. Russ. J. Electrochem. 2006, 42, 370–374. [Google Scholar]
  13. Guerrini, E.; Cristiani, P.; Grattieri, M.; Santoro, C.; Li, B.; Trasatti, S. Electrochemical Behavior of Stainless Steel Anodes in Membraneless Microbial Fuel Cells. J. Electrochem. Soc. 2013, 161, H62. [Google Scholar]
  14. Makhdoom, M.A.; Ahmad, A.; Kamran, M.; Abid, K.; Haider, W. Microstructural and Electrochemical Behavior of 2205 Duplex Stainless Steel Weldments. Surf. Interfaces 2017, 9, 189–195. [Google Scholar]
  15. Skrotzki, W.; Pukenas, A.; Odor, E.; Joni, B.; Ungar, T.; Völker, B.; Hohenwarter, A.; Pippan, R.; George, E.P. Microstructure, Texture, and Strength Development during High-Pressure Torsion of CrMnFeCoNi High-Entropy Alloy. Crystals 2020, 10, 336. [Google Scholar] [CrossRef]
  16. George, E.P.; Raabe, D.; Ritchie, R.O. High-Entropy Alloys. Nat. Rev. Mater. 2019, 4, 515–534. [Google Scholar]
  17. Koch, C.C. Nanocrystalline High-Entropy Alloys. J. Mater. Res. 2017, 32, 3435–3444. [Google Scholar] [CrossRef]
  18. Keil, T.; Utt, D.; Bruder, E.; Stukowski, A.; Albe, K.; Durst, K. Solid Solution Hardening in CrMnFeCoNi-Based High Entropy Alloy Systems Studied by a Combinatorial Approach. J. Mater. Res. 2021, 36, 2558–2570. [Google Scholar] [CrossRef]
  19. Han, Z.; Ren, W.; Yang, J.; Du, Y.; Wei, R.; Zhang, C.; Chen, Y.; Zhang, G. The Deformation Behavior and Strain Rate Sensitivity of Ultra-Fine Grained CoNiFeCrMn High-Entropy Alloys at Temperatures Ranging from 77 K to 573 K. J. Alloys Compd. 2019, 791, 962–970. [Google Scholar]
  20. Kim, D.; Chung, W.; Shin, B.-H. Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750. Metals 2023, 13, 957. [Google Scholar] [CrossRef]
  21. Shams, S.A.A.; Jang, G.; Won, J.W.; Bae, J.W.; Jin, H.; Kim, H.S.; Lee, C.S. Low-Cycle Fatigue Properties of CoCrFeMnNi High-Entropy Alloy Compared with Its Conventional Counterparts. Mater. Sci. Eng. A 2020, 792, 139661. [Google Scholar]
  22. Shin, B.-H.; Kim, S.; Park, J.; Ok, J.-W.; Kim, D.; Yoon, J.-H. Study of Precipitated Secondary Phase at 700 °C on the Electrochemical Properties of Super Duplex Stainless Steel AISI2507: Advanced High-Temperature Safety of a Lithium-Ion Battery Case. Materials 2024, 17, 2009. [Google Scholar] [CrossRef] [PubMed]
  23. Klink, J.; Hebenbrock, A.; Grabow, J.; Orazov, N.; Nylén, U.; Benger, R.; Beck, H.-P. Comparison of Model-Based and Sensor-Based Detection of Thermal Runaway in Li-Ion Battery Modules for Automotive Application. Batteries 2022, 8, 34. [Google Scholar] [CrossRef]
  24. Petit, M.; Prada, E.; Sauvant-Moynot, V. Development of an Empirical Aging Model for Li-Ion Batteries and Application to Assess the Impact of Vehicle-to-Grid Strategies on Battery Lifetime. Appl. Energy 2016, 172, 398–407. [Google Scholar]
  25. Shin, B.-H.; Park, J.; Kim, S.; Ok, J.-W.; Kim, D.-I.; Yoon, J.-H. Study of Electroless Nickel Plating on Super Duplex Stainless Steel for Lithium-Ion Battery Cases: Electrochemical Behaviour and Effects of Plating Time. Metals 2024, 14, 307. [Google Scholar] [CrossRef]
  26. Shin, B.-H.; Kim, S.; Park, J.; Ok, J.-W.; Kim, D.-I.; Kim, D.; Yoon, J.-H. Effect of Secondary Phase on Electroless Ni Plating Behaviour of Super Duplex Stainless Steel SAF2507 for Advanced Li-Ion Battery Case. Materials 2024, 17, 1441. [Google Scholar] [CrossRef] [PubMed]
  27. Shin, B.-H.; Kim, D.; Yoon, J.-H. Crystallization of Secondary Phase on Super-Duplex Stainless Steel SAF2507: Advanced Li-Ion Battery Case Materials. Crystals 2024, 14, 378. [Google Scholar] [CrossRef]
  28. Maurya, A.K.; Pandey, C.; Chhibber, R. Effect of Filler Metal Composition on Microstructural and Mechanical Characterization of Dissimilar Welded Joint of Nitronic Steel and Super Duplex Stainless Steel. Arch. Civ. Mech. Eng. 2022, 22, 90. [Google Scholar]
  29. Valeriano, L.d.C.; Correa, E.O.; Mariano, N.A.; Robin, A.L.M.; Machado, M.A.G. Influence of the Solution-Treatment Temperature and Short Aging Times on the Electrochemical Corrosion Behaviour of Uns S32520 Super Duplex Stainless Steel. Mater. Res. 2019, 22, e20180774. [Google Scholar]
  30. Ha, H.-Y.; Lee, T.-H.; Bae, J.-H.; Chun, D.W. Molybdenum Effects on Pitting Corrosion Resistance of FeCrMnMoNC Austenitic Stainless Steels. Metals 2018, 8, 653. [Google Scholar] [CrossRef]
  31. Vukkum, V.B.; Christudasjustus, J.; Darwish, A.A.; Storck, S.M.; Gupta, R.K. Enhanced Corrosion Resistance of Additively Manufactured Stainless Steel by Modification of Feedstock. Npj Mater. Degrad. 2022, 6, 2. [Google Scholar]
  32. Metikoš-Huković, M.; Babić, R.; Grubač, Z.; Petrović, Ž.; Lajçi, N. High Corrosion Resistance of Austenitic Stainless Steel Alloyed with Nitrogen in an Acid Solution. Corros. Sci. 2011, 53, 2176–2183. [Google Scholar]
  33. Lee, S.M.; Lee, W.G.; Kim, Y.H.; Jang, H. Surface Roughness and the Corrosion Resistance of 21Cr Ferritic Stainless Steel. Corros. Sci. 2012, 63, 404–409. [Google Scholar]
  34. Ha, H.-Y.; Jang, M.-H.; Lee, T.-H.; Moon, J. Interpretation of the Relation between Ferrite Fraction and Pitting Corrosion Resistance of Commercial 2205 Duplex Stainless Steel. Corros. Sci. 2014, 89, 154–162. [Google Scholar]
  35. Saravanan, P.; Govindaraj, Y.; Khalkho, B.; Srikanth, S.; Kumar, V.; Neelakantan, L. Mechanical Properties and Corrosion Behaviour of Developed High Nitrogen High Manganese Stainless Steels. Mater. Werkst. 2023, 54, 615–626. [Google Scholar] [CrossRef]
  36. Seo, M.; Hultquist, G.; Leygraf, C.; Sato, N. The Influence of Minor Alloying Elements (Nb, Ti and Cu) on the Corrosion Resistivity of Ferritic Stainless Steel in Sulfuric Acid Solution. Corros. Sci. 1986, 26, 949–960. [Google Scholar] [CrossRef]
  37. Kitta, M.; Kataoka, R. Ability of Li4Ti5O12 to Suppress Li Metal Deposition under Overpotential Conditions Confirmed by Electrochemical Surface Plasmon Resonance Spectroscopy. Int. J. Electrochem. Sci. 2023, 18, 100223. [Google Scholar]
  38. Masarapu, C.; Subramanian, V.; Zhu, H.; Wei, B. Long-Cycle Electrochemical Behavior of Multiwall Carbon Nanotubes Synthesized on Stainless Steel in Li Ion Batteries. Adv. Funct. Mater. 2009, 19, 1008–1014. [Google Scholar]
  39. Amatsuka, S.; Nishimoto, M.; Muto, I.; Kawamori, M.; Takara, Y.; Sugawara, Y. Micro-Electrochemical Insights into Pit Initiation Site on Aged UNS S32750 Super Duplex Stainless Steel. Npj Mater. Degrad. 2023, 7, 15. [Google Scholar]
  40. Paulraj, P.; Garg, R. Effect of Intermetallic Phases on Corrosion Behavior and Mechanical Properties of Duplex Stainless Steel and Super-Duplex Stainless Steel. Adv. Sci. Technol. Res. J. 2015, 9, 87–105. [Google Scholar]
  41. Topolska, S.; Łabanowski, J. Effect of Microstructure on Impact Toughness of Duplex and Superduplex Stainless Steels. J. Achiev. Mater. Manuf. Eng. 2009, 36, 142–149. [Google Scholar]
  42. Zhilyaev, A.P.; Shakhova, I.; Belyakov, A.; Kaibyshev, R.; Langdon, T.G. Wear Resistance and Electroconductivity in Copper Processed by Severe Plastic Deformation. Wear 2013, 305, 89–99. [Google Scholar] [CrossRef]
  43. Elhoud, A.M.; Renton, N.C.; Deans, W.F. Hydrogen Embrittlement of Super Duplex Stainless Steel in Acid Solution. Int. J. Hydrogen Energy 2010, 35, 6455–6464. [Google Scholar] [CrossRef]
  44. Kannan, A.R.; Shanmugam, N.S.; Rajkumar, V.; Vishnukumar, M. Insight into the Microstructural Features and Corrosion Properties of Wire Arc Additive Manufactured Super Duplex Stainless Steel (ER2594). Mater. Lett. 2020, 270, 127680. [Google Scholar]
  45. Valiente Bermejo, M.A.; Thalavai Pandian, K.; Axelsson, B.; Harati, E.; Kisielewicz, A.; Karlsson, L. Microstructure of Laser Metal Deposited Duplex Stainless Steel: Influence of Shielding Gas and Heat Treatment. Weld. World 2021, 65, 525–541. [Google Scholar]
  46. Speidel, M.O. Nitrogen Containing Austenitic Stainless Steels. Mater. Werkst. Entwickl. Fert. Prüfung Eig. Anwendungen Tech. Werkst. 2006, 37, 875–880. [Google Scholar]
  47. Baghdadchi, A.; Hosseini, V.A.; Valiente Bermejo, M.A.; Axelsson, B.; Harati, E.; Högström, M.; Karlsson, L. Wire Laser Metal Deposition of 22% Cr Duplex Stainless Steel: As-Deposited and Heat-Treated Microstructure and Mechanical Properties. J. Mater. Sci. 2022, 57, 9556–9575. [Google Scholar] [CrossRef]
  48. Acharyya, S.G.; Khandelwal, A.; Kain, V.; Kumar, A.; Samajdar, I. Surface Working of 304L Stainless Steel: Impact on Microstructure, Electrochemical Behavior and SCC Resistance. Mater. Charact. 2012, 72, 68–76. [Google Scholar]
  49. Vignal, V.; Delrue, O.; Heintz, O.; Peultier, J. Influence of the Passive Film Properties and Residual Stresses on the Micro-Electrochemical Behavior of Duplex Stainless Steels. Electrochim. Acta 2010, 55, 7118–7125. [Google Scholar]
  50. Faraji, H.; Yıldız, Ç.; Arshad, A.; Arıcı, M.; Choukairy, K.; El Alami, M. Passive Thermal Management Strategy for Cooling Multiple Portable Electronic Components: Hybrid Nanoparticles Enhanced Phase Change Materials as an Innovative Solution. J. Energy Storage 2023, 70, 108087. [Google Scholar]
  51. Yoo, Y.-R.; Choi, S.-H.; Kim, Y.-S. Effect of Laser Peening on the Corrosion Properties of 304L Stainless Steel. Materials 2023, 16, 804. [Google Scholar] [CrossRef] [PubMed]
  52. Nilsson, J.O.; Wilson, A. Influence of Isothermal Phase Transformations on Toughness and Pitting Corrosion of Super Duplex Stainless Steel SAF 2507. Mater. Sci. Technol. 1993, 9, 545–554. [Google Scholar] [CrossRef]
  53. Pettersson, N.; Pettersson, R.F.A.; Wessman, S. Precipitation of Chromium Nitrides in the Super Duplex Stainless Steel 2507. Metall. Mater. Trans. A 2015, 46, 1062–1072. [Google Scholar] [CrossRef]
  54. Hwang, B.; Lee, S.; Ahn, J. Effect of Oxides on Wear Resistance and Surface Roughness of Ferrous Coated Layers Fabricated by Atmospheric Plasma Spraying. Mater. Sci. Eng. A 2002, 335, 268–280. [Google Scholar]
  55. Bogdan, D.; Grosu, I.-G.; Filip, C. How Thick, Uniform and Smooth Are the Polydopamine Coating Layers Obtained under Different Oxidation Conditions? An in-Depth AFM Study. Appl. Surf. Sci. 2022, 597, 153680. [Google Scholar]
  56. Fredriksson, W.; Edström, K. XPS Study of Duplex Stainless Steel as a Possible Current Collector in a Li-Ion Battery. Electrochim. Acta 2012, 79, 82–94. [Google Scholar]
  57. Gasana, E.; Westbroek, P.; Hakuzimana, J.; De Clerck, K.; Priniotakis, G.; Kiekens, P.; Tseles, D. Electroconductive Textile Structures through Electroless Deposition of Polypyrrole and Copper at Polyaramide Surfaces. Surf. Coat. Technol. 2006, 201, 3547–3551. [Google Scholar]
Metals 14 00846 g001
Figure 1. Schematic diagram of the manufacturing process (#a casting, #b cold rolling, and #c annealing) of high entropy alloys (HEAs) (Cantor).
Figure 1. Schematic diagram of the manufacturing process (#a casting, #b cold rolling, and #c annealing) of high entropy alloys (HEAs) (Cantor).
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Figure 2. EBSD images showing the microstructures of Cantor (HEA) with different manufacturing processes. (a) IPF image for casting; (b) IPF image for cold rolling; (c) IPF image for annealing; (d) IPF map of the FCC structure of Cantor.
Figure 2. EBSD images showing the microstructures of Cantor (HEA) with different manufacturing processes. (a) IPF image for casting; (b) IPF image for cold rolling; (c) IPF image for annealing; (d) IPF map of the FCC structure of Cantor.
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Figure 3. Potential (V) vs. time (s) curve; open circuit potential results for different manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
Figure 3. Potential (V) vs. time (s) curve; open circuit potential results for different manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
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Figure 4. Potential (V) vs. current density (A/cm2) curve; potentiodynamic polarization curve according to the manufacturing process of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
Figure 4. Potential (V) vs. current density (A/cm2) curve; potentiodynamic polarization curve according to the manufacturing process of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
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Figure 5. Electrochemical impedance spectroscopy results for different manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution. (a) Frequency (Hz) vs. phase of Z (resistance degree of phase with frequency, °), Bode plot; (b) Frequency (Hz) vs. lZl (resistance with frequency, ohms), Bode plot; (c) Nyquist plot Zre (resistance real, ohms) vs. Zim (resistance image, ohms); (d) Circuit of EIS.
Figure 5. Electrochemical impedance spectroscopy results for different manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution. (a) Frequency (Hz) vs. phase of Z (resistance degree of phase with frequency, °), Bode plot; (b) Frequency (Hz) vs. lZl (resistance with frequency, ohms), Bode plot; (c) Nyquist plot Zre (resistance real, ohms) vs. Zim (resistance image, ohms); (d) Circuit of EIS.
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Figure 6. Current density (μA/cm2) vs. time (s) curve of casting, cold rolling, and annealing HEAs in 5.85 wt.% NaCl electrolyte solution (yellow line: Current density at which CPT is determined).
Figure 6. Current density (μA/cm2) vs. time (s) curve of casting, cold rolling, and annealing HEAs in 5.85 wt.% NaCl electrolyte solution (yellow line: Current density at which CPT is determined).
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Figure 7. Pitting morphology (black sites and yellow arrows) for the different manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution. (a) Before corrosion test; (b) Casting; (c) Cold rolling; (d) Annealing.
Figure 7. Pitting morphology (black sites and yellow arrows) for the different manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution. (a) Before corrosion test; (b) Casting; (c) Cold rolling; (d) Annealing.
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Figure 8. Counts (s) vs. binding energy (eV) curve; XPS results for the different manufacturing processes of HEAs. (a) Cr2p3; (b) O 1s.
Figure 8. Counts (s) vs. binding energy (eV) curve; XPS results for the different manufacturing processes of HEAs. (a) Cr2p3; (b) O 1s.
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Figure 9. Breakdown of the passivation layer and pitting growth according to the manufacturing process of HEAs (Cantor) in 3.5 wt.% NaCl electrolyte solution. (a) Casting; (b) Cold rolling; (c) Annealing.
Figure 9. Breakdown of the passivation layer and pitting growth according to the manufacturing process of HEAs (Cantor) in 3.5 wt.% NaCl electrolyte solution. (a) Casting; (b) Cold rolling; (c) Annealing.
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Table 1. Chemical composition based on manufacturing condition of HEA by EDS (wt.%).
Table 1. Chemical composition based on manufacturing condition of HEA by EDS (wt.%).
PositionCrMnFeCoNiO
(a) Casting surface18.8 ± 0.919.0 ± 1.119.4 ± 0.619.7 ± 0.420.0 ± 0.33.1 ± 1.4
(b) Cold rolling surface18.5 ± 0.818.9 ± 1.119.2 ± 0.519.5 ± 0.419.8 ± 0.44.1 ± 1.5
(c) Annealing surface18.9 ± 0.618.9 ± 1.219.3 ± 0.520.0 ± 0.320.3 ± 0.22.6 ± 1.4
Table 2. Values of potential for major alloys and for manufacturing processes of HEAs.
Table 2. Values of potential for major alloys and for manufacturing processes of HEAs.
ConditionCastingCold RollingAnnealingCrMnFeCoNi
Potential, V0.120.080.18−0.56−1.05−0.44−0.28−0.23
Table 3. Key values from the potentiodynamic polarization curve for the manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
Table 3. Key values from the potentiodynamic polarization curve for the manufacturing processes of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
Major pointEcorrIcorrEpit
(a) Casting0.05 V2 × 10−7 A/cm20.25 V
(b) Cold rolling−0.03 V9 × 10−7 A/cm2X
(c) Annealing0.14 V3 × 10−5 A/cm20.42 V
Table 4. Major values according to the manufacturing process of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
Table 4. Major values according to the manufacturing process of HEAs (Cantor) with 3.5 wt.% NaCl electrolyte solution.
SpecimenRsCPERp
Np
(a) Casting6.1 ohms7.2 × 1030.856.1 kohms
(b) Cold rolling6.1 ohms6.1 × 1030.855.2 kohms
(c) Annealing6.1 ohms13.3 × 1030.8511.3 kohms
Table 5. Chemical composition at the different positions in Figure 7 after corrosion test of annealed HEA, determined by EDS (wt.%).
Table 5. Chemical composition at the different positions in Figure 7 after corrosion test of annealed HEA, determined by EDS (wt.%).
PositionCrMnFeCoNiOCl
114.4 ± 1.116.2 ± 0.917.1 ± 0.917.9 ± 0.918.1 ± 1.313.1 ± 0.73.2 ± 0.8
214.2 ± 1.116.5 ± 0.816.9 ± 0.917.3 ± 0.818.0 ± 1.113.6 ± 0.93.5 ± 0.7
312.9 ± 1.314.3 ± 2.115.5 ± 1.416.9 ± 1.517.8 ± 1.517.2 ± 1.15.4 ± 1.2
413.8 ± 1.414.1 ± 1.815.7 ± 2.416.5 ± 1.317.1 ± 1.817.9 ± 1.14.9 ± 1.4
516.6 ± 0.817.5 ± 0.917.7 ± 1.117.9 ± 1.118.6 ± 0.610.1 ± 0.7.1.6 ± 0.7
616.5 ± 0.917.1 ± 1.217.5 ± 1.218.1 ± 1.018.5 ± 0.810.9 ± 0.61.4 ± 0.7
Table 6. Pitting resistance equivalent number and critical pitting temperature of HEAs (casting, cold rolling, and annealing) and stainless steel (AISI304 and AISI316) for comparison of their corrosion resistance.
Table 6. Pitting resistance equivalent number and critical pitting temperature of HEAs (casting, cold rolling, and annealing) and stainless steel (AISI304 and AISI316) for comparison of their corrosion resistance.
Condition(a) Casting(b) Cold Rolling(c) AnnealingAISI304AISI316
PREN18.518.518.518.221.5
CPT14 °C12 °C19 °C1 °C22 °C
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Lim, J.; Shin, B.-H.; Kim, D.-I.; Bae, J.-S.; Ok, J.-W.; Kim, S.; Park, J.; Lee, J.I.; Yoon, J.-H. Effect of Annealing after Casting and Cold Rolling on Microstructure and Electrochemical Behavior of High-Entropy Alloy, Cantor. Metals 2024, 14, 846. https://doi.org/10.3390/met14080846

AMA Style

Lim J, Shin B-H, Kim D-I, Bae J-S, Ok J-W, Kim S, Park J, Lee JI, Yoon J-H. Effect of Annealing after Casting and Cold Rolling on Microstructure and Electrochemical Behavior of High-Entropy Alloy, Cantor. Metals. 2024; 14(8):846. https://doi.org/10.3390/met14080846

Chicago/Turabian Style

Lim, Jinsurang, Byung-Hyun Shin, Doo-In Kim, Jong-Seong Bae, Jung-Woo Ok, Seongjun Kim, Jinyong Park, Je In Lee, and Jang-Hee Yoon. 2024. "Effect of Annealing after Casting and Cold Rolling on Microstructure and Electrochemical Behavior of High-Entropy Alloy, Cantor" Metals 14, no. 8: 846. https://doi.org/10.3390/met14080846

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